There are known onboard acoustic devices for assisting smooth conversation within a closed space where cyclic noise occurs, such as in the cabin of a vehicle, for example (see Japanese Unexamined Patent Application Publication No. 2002-51392). The onboard acoustics device according to Japanese Unexamined Patent Application Publication No. 2002-51392 drives speakers disposed to the front of the vehicle cabin by a microphone disposed to the back of the vehicle cabin, and drives speakers disposed to the back of the vehicle cabin by a microphone disposed to the front of the vehicle cabin, thereby assisting smooth conversation among passengers in the front seat and the back seat.
On the other hand, there is also known active noise prediction technology, where cancellation sound having an antiphase as to the phase of the noise is generated from a speaker to reduce invasive noise to the cabin such as engine noise and the like, thereby actively reducing noise within the cabin (see Japanese Unexamined Patent Application Publication No. 7-30994).
However, attempting to apply a simple combination of the conversation assisting technology for within a closed space according to Japanese Unexamined Patent Application Publication No. 2002-51392 and the active noise reduction technology according to Japanese Unexamined Patent Application Publication No. 7-30994 results in a composite sound of the audio for conversation assistance and the cancellation sound for active noise reduction being output from the speaker. Since the active noise reduction technology according to Japanese Unexamined Patent Application Publication No. 7-30994 performs active noise reduction control based on the compounded sound picked up by a microphone, the behavior of this control may become unstable.